Multiple times, throughout history, paintings have sold to eager buyers only for them to later discovered they'd purchased a fake.

In fact, a museum in France found out that nearly half of its collection was fake and an art dealer in New York admitted to selling 60 forged artworks.

A famous portrait of William Shakespeare titled the "Flower portrait" turned out to be a fake.

A rare work of art can rack up a hefty price tag. But when some experts estimate that about 20% of paintings owned by major museums may not be authentic, it can be tough to know if what you're paying for is the real deal.

Below we've listed some of the most notorious art scams in history where the owners thought they had something authentic but found out their work was forged.

1/

A museum in France found out that nearly half of its collection was fake.

An autoportrait of the French painter Etienne Terrus is displayed at the museum dedicated to the artist, in Elne (Elna), on April 28, 2018. - Sad inauguration on April 27, 2018 for the museum dedicated to Etienne Terrus, in Elne, in the Pyrenees-Orientales, which saw its collection amputated by 60%, 82 paintings on 140 were counterfeitRAYMOND ROIG / AFP/Getty Images

The museum launched an investigation and subsequently confirmed, according to The Telegraph, that that painting was not the only apparent fake. In fact, they found that 82 of the 140 works in the museum were not by Terrus.

The state-owned museum had apparently bought many of the paintings for £140,000, or about $183,000, over the years (NPR put the number at $200,000), while others were given as gifts. The city's mayor Yves Barniol called it "a catastrophe," according to The Telegraph.

The city launched formal complaints and police attempted to track down the source of the forgery, but never solved the mystery.

2/

An entire New York art gallery was closed over allegations of selling fraudulent paintings.

Knoedler & Company art gallery, which had been around for 165 years, closed after allegations of selling fraudulent paintings surfaced. Investigators discovered that paintings supposedly done by classical artists were actually made by one person in Queens, New York, according to The Washington Post.

The Washington Post identified Pei Shen Qian as the alleged artist of the forged painting, though he told them he knew nothing about the frauds.

The frauds resulted in various lawsuits, including one from Domenico De Sole, chairman of the board at Sotheby's Auction House, after he and his family spent $8.3 million on a painting claiming to be by the artist Mark Rothko.

3/

An artist named Han van Meegeren sold his paintings passed off as 17th-century works.

The faux Vermeer "The last supper" (1940-1941) painted by Dutch master forger Han van Meegeren (1889-1947) is seen in this picture and displayed at the De Kunsthal gallery in Rotterdam.REUTERS/Jasper Juinen

Van Meegeran produced works which he then aged in a pizza oven to make them appear authentic, according to NPR. His work fooled many people, including Nazi top brass Hermann Goering. He made the equivalent of $30 million off of these paintings, according to NPR.

Steve Martin was apparently duped by a piece of forged art.

Steve Martin was not suspected of any wrongdoing.
Rabbani and Solimene Photography/Getty Images

Actor Steve Martin bought a painting in 2004, apparently believing it to be by the artist Heinrich Campendonk, called "Landschaft mit Pferden" or "Landscape With Horses." The New York Times reported that he paid 700,000 euros for it, which they estimated at about $850,000 at the time.

But, after Martin sold the work at a Christie's auction in 2006 for 500,000 euros, investigators found out it was likely created by Wolfgang Beltracchi, who has been called "forger of the century."

Martin, for his part, told The New York Times he had no idea and he was not accused of any wrongdoing.

"It wasn't clear that it was a fake until after Christie's had sold the picture, it was a long time after that, that it became known," he told them.

The exhibition of Modigliani was shut down early after art critic and collector Carlo Pepi alerted the authorities that the paintings were likely not authentic.

"A Michelangelo is a Michelangelo. A Picasso is a Picasso. But when a painting is a fake, it is missing its soul, and these were missing that three-dimensional elegance of Modigliani — even a child could see these were crude fakes," he told the Telegraph.

But his ruse went off too well and people were overjoyed to have them back. Malskat eventually tried to make known that the works were not restored, but forged. But, according to NPR, no one believed that the work was actually his, so he had to take extreme measures.

Still, no one believed him until he pointed to two dead giveaways hidden in the works: one included a turkey, which wouldn't have appeared in Germany during the medieval times and a portrait of the actress Marlene Dietrich, who, obviously, wasn't alive back then.

8/

“The Faun" turned out to be forged.

When the Art Institute of Chicago purchased the sculpture "The Faun" in the '90s, they thought it was created by artist Gauguin in the 1800s. But, as the New York Times reported, it was actually shown to be a forged piece of artwork created the same decade in which it was purchased.

It turns out the piece was created by the Greenhalgh family of Bolton, England and was one of a suspected 120 fakes created by the family, which raked in $20 million, according to the Times. The Art Institute declined to tell the Times how much they paid for "The Faun."

John Myatt forged more than 200 works.

Artist John Myatt made headlines in 1995 when he was arrested on charges of forging art in the style of Monet, Chagall, and Giacometti, according to The Sydney Morning Herald.

His works, which made it to galleries and auction houses like Sotheby's, and Christie's, only made him an estimated $165,000 throughout his time forging, The New York Times later reported.

He was released after serving four months of a year-long sentence and today sells "genuine fakes" legally, according to the Sydney Morning Herald, which he claims are much better than the forged pieces he used to do.

"The fakes I'm doing now are much better than the fakes I ever did back then," he told them.

A portrait of William Shakespeare turned out to be a fake.

The painting was supposedly created in 1609, but the National Portrait Gallery told the BBC that when they x-rayed it, they found that the paint used was created sometime in the 19th century.

"We now think the portrait dates back to around 1818 to 1840, exactly the time when there was a resurgence of interest in Shakespeare's plays," the gallery's 16th century curator Tarnya Cooper told the BBC.

But just because it's not what they thought doesn't mean it's not valuable, Royal Shakespeare Company curator David Howells told the BBC.

"Now we know the truth we can put the image in its proper context in the history of Shakespearean portraiture, alongside the other fascinating pieces in our collection in Stratford," he told them.